The attacker mowed people down along Westminster Bridge with a vehicle and attacked one policeman with a long knife before being shot dead; Benjamin Hall reports for 'Special Report'

Five people, including a London police officer who was stabbed and the alleged assailant, were killed in a terror attack that saw more 40 people injured outside the Parliament building on Wednesday in an act described as "sick and depraved" by British Prime Minister Theresa May.

Acting Metropolian Police Deputy Commissioner and Head of Counter Terrorism Mark Rowley said there was only one attacker who authorities believe was "inspired by international terrorism."

Police said a vehicle mowed down pedestrians on London's Westminster Bridge, leaving more than a dozen with injuries described as catastrophic.

Rowley said the car then crashed near to Parliament, and one man - armed with a knife - continued the attack and tried to enter Parliament.

The knife-wielding attacker stabbed a police officer and was shot on the grounds outside Britain's Parliament, sending the compound into lockdown for hours.

The threat level for international terrorism in the U.K. was already listed at severe, and will remain so, according to May. The attack came on the one-year anniversary of the terror attacks in Brussels in which 32 people were killed and more than 300 injured.

ISIS claimed responsibility for the coordinated suicide bombings last year -- two at the Brussels Airport in Zaventem and one at Maalbeek metro station in central Brussels. The bombings were the deadliest terror attack in Belgium's history.

ISIS has long promoted the use of vehicles and knives in attacks by so-called "lone wolf" terrorists, particularly in Western countries. Senior U.S. officials expected an uptick of terror attacks in Europe as they ramp up the fight to take ISIS capital Raqqa, Fox News is told.

The terrorist group has not claimed responsibility for Wednesday's attack, but a European government official told Reuters that investigators are considering the possibility the attack was inspired by ISIS propaganda.

London police were called to the Parliament building at around 2:40 p.m. local time (10:40 a.m. EST) after reports of a vehicle crashing into a crowd at Westminster Bridge near Britain's parliament.

Rick Longley told the Press Association that he heard a bang and saw a car plow into pedestrians and come to a crashing stop. Images from the scene showed pedestrians sprawled on the ground, with blood streaming from a woman surrounded by a scattering of postcards.

"They were just laying there and then the whole crowd just surged around the corner by the gates just opposite Big Ben," he said. "A guy came past my right shoulder with a big knife and just started plunging it into the policeman. I have never seen anything like that. I just can't believe what I just saw."

The former Polish foreign minister Radoslaw Sikorski posted a video on Twitter that seems to show people lying injured in the road on Westminster Bridge.

Sikorski, a senior fellow at the Harvard Centre for European Studies, says he saw at least five people lying on the ground after being "mown down" by a car.

Sikorski told the BBC he "heard what I thought what I thought was just a collision and then I looked through the window of the taxi and someone down, obviously in great distress.

"Then I saw a second person down, and I started filming, then I saw three more people down, one of them bleeding profusely."

Daily Mail journalist Quentin Letts said he saw a man in black attack a police officer outside Parliament before being shot two or three times as he tried to storm into the House of Commons.

"He had something in his hand, it looked like a stick of some sort, and he was challenged by a couple of policemen in yellow jackets," Letts told the BBC. "And one of the yellow-jacketed policemen fell down and we could see the man in black moving his arm in a way that suggested he was stabbing or striking the yellow-jacketed policeman."

Dennis Burns, who was just entering Parliament for a meeting, told the Press Association he heard a radio message saying an officer had been stabbed. Police and security rushed outside as he was going in.

"When I got inside I was wondering what the hell was going on and I saw dozens of panicked people running down the street," he said. "The first stream was around 30 people and the second stream was 70 people. It looked like they were running for their lives."